This invention relates to a motor vehicle electrical system, and more particularly to a high voltage protection circuit that isolates an electrical load from damage and/or power interruption during over-voltage conditions.
It is well known that motor vehicle electrical systems are subject to over-voltage under a number of different conditions. For example, jump-starting the engine with an excessive supply voltage subjects the electrical loads to the excessive voltage as well. Also, the ignition voltage is subject to transient surges during so-called load dump events when the vehicle storage battery is momentarily or permanently disconnected from an engine-driven alternator. In such case, the alternator output voltage can rise well above the nominal charging voltage before the voltage regulator can remove the alternator field winding excitation. For this reason, many load devices are provided with over-voltage protection circuits that either block high voltages or shut down when the supply voltage rises above an over-voltage threshold. This is obviously an undesirable condition since it adds to the cost of the individual load devices, and in the case of a shut-down, renders the load device inoperative until a reset occurs. Accordingly, what is needed is an over-voltage protection apparatus that protects electrical load devices from damage and/or power interruption during both improper jump-starting and load dump conditions.
The present invention is directed to an improved over-voltage protection circuit for a motor vehicle electrical system, including an over-voltage responsive circuit for momentarily disconnecting the vehicle storage battery and alternator from vehicle electrical loads, and an auxiliary storable battery for supplying a safe operating voltage to the electrical loads during the momentary disconnection. The over-voltage responsive circuit includes a MOSFET device that couples the vehicle storage battery and alternator to the electrical loads and auxiliary storage battery, and a voltage responsive circuit that turns the MOSFET device off to decouple the electrical loads and auxiliary storable battery from the vehicle storable battery and alternator so long as an over-voltage condition is detected.